This invention relates to the art of blow molding from a parison of articles of organic plastic material susceptible to the improvement of their properties by orientation and has for its principal object maintaining a rapid operating cycle despite the fact that the particular plastic may require prolonged tempering cycles to obtain conditions suitable for orientation.
The art teaches various methods and apparatus for obtaining blow molded articles of organic plastic material from a parison, such as U.S. Pat. No. 3,349,155 and Re. 27,104. Generally, these methods are characterized by forming a parison in a parison mold on a blow core, placing said formed parison and blow core into a blow mold and expanding said parison in the blow mold by means of fluid pressure.
While the blow molding operation tends to impart orientation to the article, such orientation is predominantly circumferential, i.e., not bi-axial. Also, the degree of such orientation is difficult to control. It is therefore difficult to obtain the advantageous properties in the article that bi-axial orientation by stretching and blowing is capable of providing.
It is known that the control of orientation depends largely upon the control of the temperature of the parison just prior to orientation. It is found that such temperature control is best obtained by enclosing the article prior to the orienting step in cooling and/or heating means that impart temperatures, preferably by contact with the corresponding surfaces of the parison, to the regions of the parison corresponding to the degree of deformation that is intended for such regions.
However, certain plastics, polypropylene being the most prominent, require a prolonged tempering cycle prior to bi-axial orientation by stretching and blowing. Thus, tempering cycles in terms of minutes may be called for, in contrast to tempering cycles measured in seconds that are needed in connection with other materials.
It is, therefore, a principal objective of the present invention to devise a method and apparatus for use with this type of material which may introduce an extended tempering cycle.
It is a particular object of this invention to provide a method and apparatus as aforesaid which does not prolong the actual operating cycle of the device.
It is a still further object of the present invention to provide a method and apparatus as aforesaid and which is capable of providing the aforesaid advantages without the necessity of constructing an extremely bulky and inefficient heating apparatus.
Further objects and advantages of the present invention will appear hereinbelow.